Do not coerce session keys to strings for getters/setters

[radar]

I came across a bug today where I had this code in my action's test:

  let(:params) do
    {
      book_permalink: "book-permalink",
      chapter_permalink: "chapter-permalink",
      note_id: 1,
      text: "Updated note text",
      "rack.session" => { user_id: 1 },
    }
  end

But then when attempting to access the session's value in the action's code:

request.session[:user_id] => nil

This is because the previous implementation was stringifying keys. There wasn't a session key stored as "user_id" in the session, so it returns nil.

This PR removes the cooercing-to-string of these values. So whatever is put in there is now maintained in the data type it was insertedin.

[parndt]

Nice; I've definitely had this issue in the past.

[timriley]

Thanks for this, @radar! I think this is a good change. One thing I want to do with this is have a closer poke at the ramifications for our Rack 2/3 compatibility. This string-key-ifying session wrapper was introduced in #477 as part of our Rack 3 compatibility effort, and I just want to be doubly sure that we're not introducing any regressions (the fact that our tests are happy is a good sign!).